The invention is based on a fuel injection pump for internal combustion engines.
In a fuel injection pump of this kind, known from German Offenlegungsschrift No. 31 18 669, the position of a first control edge, which beyond a predetermined stroke opens the relief conduit leading to the reservoir, determines the quantity of fuel to be displaced into the reservoir, which following the simultaneously occurring end of the fuel injection by means of the pumping stroke of the pump piston is pumped by this pump piston over the remaining course of its stroke. During the intake stroke that follows, this fuel quantity is supposed to be drawn out of the reservoir once again, until the control edge again closes the relief conduit, at the same pump piston stroke position. The metering of the fuel injection quantity via a magnetic valve in the fuel supply conduit is effected by the opening up of the mouth of this conduit into the pump work chamber with the front edge of the pump piston, which happens only at the end of the intake stroke. In this pump piston position, the pressure in the pump work chamber has dropped to the vapor pressure value of the fuel. In the known apparatus, the control edge that controls the relief conduit is an oblique control edge disposed on the pump piston, and the pump piston can be rotated in order to vary the instant of the opening action. The end of injection can thus be controlled, and the injection onset can also be controlled, in combination with the fuel metering quantity that has been appropriately corrected as needed, in such a way that the injection phase takes place in an arbitrary portion of the pump piston pumping stroke. This apparatus has the disadvantage, however, that the diversion of the fuel into the reservoir and its refilling from the reservoir into the pump work chamber takes place at entirely different pressure levels, which has a substantial influence on the quantity of fuel that is actually returned to the pump work chamber. In other words, the quantity returned to the pump work chamber is not exactly the quantity that is pumped by the pump piston over the remaining course of its stroke, so that this fact too must be taken into consideration in metering the quantity of fuel that is to be injected. The error is a product of the various pressure drops at the cross section of the mouth of the relief conduit during the diversion on the one hand and then during the refilling operation on the other. The length of time required for discharging the reservoir is a further substantial influence, so that there is a factor of rpm dependency as well. The reservoir must also, for safety's sake, have a pressure limiting valve in order to avoid an overload, since for the above-stated reason pumping takes place continuously.
In the known apparatus, there are accordingly influences on the injection onset and the supply quantity that must be compensated for by an electrical control unit in a very complicated manner by means of appropriate correction values.
An embodiment is known from German Offenlegungsschrift No. 28 41 807. Here, in order to control the inlet in a distributor injection pump for example, a longitudinal groove is disposed on the rotating pump piston and the reservoir is disposed in the fuel supply conduit downstream of a check valve and connected with the control location of the relief conduit by means of a control edge provided on an annular slide that is displaceable on the pump piston. This control edge serves to set the fuel injection quantity, because after a substantially constant onset of supply by the pump piston the control edge opens the work chamber toward the reservoir beyond a supply stroke that is dependent on the position of the control edge. The delivery of fuel into the work chamber takes place via a variable cross section downstream of the check valve; however, this cross section functions in accordance with the required fuel quantity that is determined by the position of the annular slide. As a result of the operation of the variable cross section, in this embodiment, the ratio of the fresh air quantity to the fuel quantity to be injected is also kept constant by means of the adaptation of the recirculated exhaust gas quantity.